Have you checked your Lyman bowl to make sure it's tight on the base? It could be loose causing the bowl to not vibrate correctly?
rg1: Good idea ... so I checked the bowl, and it is indeed tight. It really just seems like the springs are worn out, so the cyclic action isn't as fast as it used to be. Or maybe it's the motor ... it exceeds my engieering knowledge.
So far, nobody mentioned a tumbler they really didn't like. It's a tough call to make.